NORMAL HISTOLOGY. 



cesses are also present. The small bipolar ganglion-cells, the 

 preponderating elements of the zone, send their axis-cylinder pro- 

 cesses into the inner reticular layer, their protoplasmic processes 

 extending as far as the outer reticular layer, where they terminate in 

 fine fibrillae, taking part in the subepithelial net-work. It is 

 probable that fibrils ascend from this reticulum between the rods and 

 the cones. The additional nuclei in this zone belong to the radial 

 supporting fibres. 



The outer reticular layer appears as a narrow zone made up of 

 an intricate net-work of fine fibres with sparingly distributed 

 nuclei. The fibrillae are derived from the neuroglia and from the 

 subdivision of the processes of nerve-cells, whose branched pro- 

 toplasmic extensions are distributed to this layer, while their 

 axis-cylinder processes penetrate as far as the inner reticular zone. 



The outer nuclear layer and the layer of rods and cones, the 

 remaining strata of the inner lamina of the retina, together constitute 

 the neuro-epithelium. Since the rods and the cones and the outer 

 nuclear layer are parts of a single lamina of tall neuro-epithelial 

 elements, the visual cells, of which they are respectively the outer 

 and inner segments, these strata must be regarded as subdivisions of 

 the one broad zone, and not as independent retinal layers. The 

 outer and inner segments are sharply separated by the intervening 

 membrana limitans, through the openings in which the rods and 

 the cones protrude. The constituents of the neuro-epithelium 

 are, therefore, the rod-visual cells and the cone-visual cells, 

 supported by the sustentacular tissue. 



The rod-visual cells are composed of two parts, the one situ- 

 ated without the limitans, including the non-nucleated and highly- 

 specialized segments, the rods, and the other within the limitans, 

 consisting of slender varicose elements, the rod-fibres, provided 

 with fusiform enlargements, the rod-spherules, which contain the 

 nuclei of the visual cells. The rods are slender cylindrical struct- 

 ures, about 60 fji in length and 2 fj. in breadth, composed of two 

 chemically and optically distinct parts, the outer and inner seg- 

 ments. 



The outer segments of the rods are cylindrical, apparently 

 homogeneous, highly-refracting bodies, which, after certain reagents, 

 exhibit a disposition to break up into thin transverse disks. The 

 outer segments of the rods are further distinguished as being the 

 exclusive seat of the peculiar visual purple or rhodopsin. The 

 inner segments of the rods are slightly broader and less regularly 

 cylindrical, and present a finely granular appearance, the parts of 

 the segments nearest the membrana limitans possessing a peripheral 

 longitudinal striation. 



